1. Field of the Invention
The present invention relates to a low resistance electrode and a compound semiconductor light emitting device including the same, and more particularly, to a low resistance electrode and a compound semiconductor light emitting device including the same in which reflectivity is improved by preventing agglomeration.
2. Description of the Related Art
A semiconductor light emitting device converts an electric signal into light using properties of compound semiconductor device, such as a light emitting diode (LED). Compared with other illuminants, a semiconductor light emitting device has a long lifetime, a low driving voltage, and low power consumption. Also, a semiconductor light emitting device has excellent response speed and impact resistance and can be manufactured to be small-sized and lightweight. A semiconductor light emitting device can generate light at different wavelengths, depending on the semiconductor material. Thus, if necessary, light of different wavelengths can be generated and used. Specifically, with the development of production technology and improvements in device structure, high-brightness semiconductor light emitting devices have been developed and their range of applications have increased. Further, since high-brightness semiconductor light emitting devices that emit blue light were developed in the middle of the 1990's, true colors can be reproduced using red, green and blue high-brightness semiconductor light emitting devices.
FIG. 1 is a schematic view illustrating an operating principle of a conventional LED. Referring to FIG. 1, the LED includes a semiconductor light emitting device 10 formed on a sapphire substrate 40, a p-type electrode 20 formed on the semiconductor light emitting device 10, and an n-type electrode 30 formed at one corner of the semiconductor light emitting device 10. If a forward voltage is applied to the LED electrodes 20 and 30, recombination of holes from a p-type clad layer 17 and electrons from an n-type clad layer 13 in an active layer 15 results in light emission. The light that is emitted from the active layer 15 is reflected by the p-type electrode 20 and emitted out of the LED through the sapphire substrate 40. In such an LED, since the p-type electrode 20 must reflect the light, a conductive metal having a high reflectivity, such as Ag, is used as the p-type electrode 20.
A semiconductor having a large direct bandgap energy (about 2.8 eV or more) is necessary for emitting blue light. Semiconductor devices that emit blue and green light using ternary materials of groups II-VI have been developed. However, their applications are limited because of relatively short operating times. Recently, group III-V semiconductor devices that emit blue light are developed. Among them, group III nitride materials (mainly, GaN related compounds) are very stable in optical, electrical and thermal stimuli and have high luminous efficiency. Thus, the group III nitride materials are often used.
FIG. 2 illustrates a conventional p-type electrode 20, which is formed on the p-type clad layer (p-type nitride semiconductor) 17 of the nitride semiconductor light emitting device 10 in a LED that uses a group-III nitride semiconductor, such as GaN, as a semiconductor light emitting device. As described above, the p-type reflective electrode 20, which may be composed of Ag, is formed on the p-type nitride semiconductor 17. In the process of forming the p-type reflective electrode 20 on the p-type nitride semiconductor 17, an electrode is deposited on a p-type nitride semiconductor and an annealing process is performed to reduce resistance.
However, there is a large difference between surface energies of the nitride semiconductor and of the metal material used as the reflective electrode. Due to such a difference in the surface energy, it is generally known that agglomeration occurs in the Ag electrode during the annealing process, as illustrated in FIGS. 4A and 4C. FIGS. 4A and 4C are top and side views of the Ag electrode in which the agglomeration occurs, respectively. If such agglomeration occurs in the Ag electrode, the reflectivity of the Ag electrode is degraded, thereby reducing the overall optical power of the LED.